cube(2): CUBE_OP_PUT takes a class mask

The mask is the writer's and is stamped once, at the moment the record's
class is known for certain; every later reader is spared re-deriving it. It
means nothing to this side — which bits are which class is a vocabulary's
business, and a kernel that interpreted one would be inventing a vocabulary.

0 is "no class", which is what every record written before the field existed
reads as, so a caller that does not classify is not writing a special value.
A store whose image is the legacy packed layout has no field to put a mask in
and drops it: that layout cannot carry a class, and saying otherwise would be
a lie about the bytes on disk.

The field is appended, so sizeof(cube_args) grows from 80 to 88 — still
distinct from the other three argument blocks, which is what the size-first
dispatch depends on.

drivers/cube/cube_syscall.c

@@ -30,7 +30,7 @@
  * encoding, which must produce exactly the key a userspace reader decodes.
  */
 int cubelinux_kernel_put(const __u8 *space, __u64 x, __u64 y, __u64 z,
-			 const void *value, size_t len);
+			 const void *value, size_t len, __u16 flags);
 ssize_t cubelinux_kernel_get(const __u8 *space, __u64 x, __u64 y, __u64 z,
 			     void *buf, size_t len);
 int cubelinux_kernel_del(const __u8 *space, __u64 x, __u64 y, __u64 z);
@@ -188,7 +188,8 @@ static long cube_args_op(unsigned int op, void __user *uargs)
 			break;
 		}
 		ret = cubelinux_kernel_put(args.coord.space, args.coord.x,
-					   args.coord.y, args.coord.z, buf, args.len);
+					   args.coord.y, args.coord.z, buf, args.len,
+					   args.flags);
 		break;
 
 	case CUBE_OP_GET: {